The principle governing the operation of most magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve where the resistance increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
The key elements of a spin valve are illustrated in FIG. 1. They are seed layer 11 (lying on lower conductive lead 10) on which is antiferromagnetic layer 12 whose purpose is to act as a pinning agent for a magnetically pinned layer. The latter is a synthetic antiferromagnet formed by sandwiching antiferromagnetic coupling layer 14 between two antiparallel ferromagnetic layers 13 (AP2) and 15 (AP1).
Next is a non-magnetic spacer layer 16 on which is low coercivity (free) ferromagnetic layer 17. A contacting layer such as lead 18 lies atop free layer 17. Not shown, but generally present, is a capping layer between 17 and 18. When free layer 17 is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will stay at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field.
If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers suffer less scattering. Thus, the resistance in this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase. The change in resistance of a spin valve is typically 8-20%.
Earlier GMR devices were designed so as to measure the resistance of the free layer for current flowing parallel to its two surfaces. However, as the quest for ever greater densities has progressed, devices that measure current flowing perpendicular to the plane (CPP), as exemplified in FIG. 1, have also emerged. CPP GMR heads are considered to be promising candidates for the over 100 Gb/in2 recording density domain (see references 1-3 below).
A related effect to the GMR phenomenon described above is tunneling magnetic resistance (TMR) in which the layer that separates the free and pinned layers is a non-magnetic insulator, such as alumina or silica. Its thickness needs to be such that it will transmit a significant tunneling current. Typically, this is around 5-20 Angstroms.
A MTJ (magnetic tunnel junction) is readily visualized by substituting a very thin dielectric layer for spacer layer 16 described above for the GMR device. The principle governing the operation of the MTJ in magnetic read sensors is the change of resistivity of the tunnel junction between two ferromagnetic layers when it is subjected to a bit field from magnetic media. When the magnetizations of the pinned and free layers are in opposite directions, the tunneling resistance increases due to a reduction in the tunneling probability. The change of resistance is typically 40%, which is much larger than for GMR devices.
With the decrease of AlOx thickness in TMR sensor, MR ratio drops with areal resistance. The present invention discloses how a higher MR ratio can be obtained while still retaining the same R.A (resistance.area product) as before. Additionally, the invention allows several of the magnetic properties of the device to be adjusted.
We note the following references to be of interest:
    1. Hitoshi Kanai “Multilayer spin valve magneto-resistive effect magnetic head with free magnetic layer including two sublayers and magnetic disk drive including same”, U.S. Pat. No. 5,896,252 (Apr. 20, 1999).    2. Dian Song et al., “Demonstrating a Tunneling Magneto-Resistive Read Head”, IEEE Transactions On Magnetics, Vol. 36, No. 5, 2000, p 2545.    3. Ishiwata et al., “Tunneling magnetoresistance transducer and method for manufacturing the same”, U.S. Pat. No. 6,452,204 (Sep. 17, 2002).    4. Hitoshi Kanai “Multilayer spin valve magneto-resistive effect magnetic head with free magnetic layer including two sublayers and magnetic disk drive including same”, U.S. Pat. No. 5,896,252 (Apr. 20, 1999)    5. K. Ohashi et al. “Low-Resistance Tunnel Magnetoresistive Head” IEEE Transactions On Magnetics, Vol. 36, NO. 5, 2000, p 2549.    6. P. P: Freitas et al., “Spin-dependent Tunnel Junctions for Memory and Read-Head applications”, IEEE Transactions On Magnetics, Vol. 36, No. 5, 2000, p 2796.    7. Ishiwata et al., “Tunneling magnetoresistance transducer and method for manufacturing the same”, U.S. Pat. No. 6,452,204 (Sep. 17, 2002).
Composite free layers, such as Co90Fe10/Ni80Fe20, are standard structures for giant magnetoresistive (GMR) sensors of magnetic recording heads. A composite free layer usually comprises two magnetic layers, namely first free layer (FL1) and second free layer (FL2), which are directly magnetic-coupled. FL1 (usually Co-rich alloys) provide strong spin dependent scattering for higher signal, while FL2 (usually Permalloy-type NiFe material) provides magnetic softness to suppress magnetic noise. This composite free layer structure has also been used in tunneling magnetoresistive (TMR) sensors, a typical TMR structure being as follows:
Buffer layer/Antiferromagnetic layer/CoFe/Ru/CoFe/AlOx/CoFe/NiFe/Capping layer
in which the Cu spacer layer of GMR structure is replaced by an AlOx layer. Compared to GMR, the advantage of TMR is higher MR ratio, which is usually larger than 20%. TMR sensors are operated in CPP (current perpendicular to plane of film) mode so that the sensor resistance will increase as the TMR sensor is scaled down in order to achieve higher recording density.
To maintain a useful sensor resistance range, the AlOx thickness has to be reduced to less than 7 Å to achieve low areal resistance, R.A, (of several ohm*μm2). However, as a consequence, the MR ratio will also drop as R.A decreases. Thus, one of the major challenges facing the TMR sensor is how to improve the MR ratio while still keeping R.A low.
Additionally, reference is made to HT04-015 (application Ser. No. 10/854,651 filed May 26, 2004) which deals with a similar problem towards which the present invention takes a different approach as well as to the following references that were found during a routine search:
U.S. Pat. No. 6,529,353 (Shimazawa et al), U.S. Pat. No. 5,896,252 (Kanai), and U.S. Pat. No. 6,519,124 (Redon et al) disclose free layers comprising CoFe and NiFe. U.S. Patent Application 2004/0047190 (Odagawa et al) describes a Ni-rich free layer. U.S. Patent Application 2004/0184198 (Tetsukawa et al) discloses a free layer comprising FeNi with Fe 5-40%. U.S. Patent Application 2004/0109263 (Suds et al) teaches a free layer comprising FeNi and a Co alloy with no disclosure of the percentage of Fe.